/compiler/nativeGen/RegAlloc/Graph/SpillCost.hs

module RegAlloc.Graph.SpillCost (
        SpillCostRecord,
        plusSpillCostRecord,
        pprSpillCostRecord,

        SpillCostInfo,
        zeroSpillCostInfo,
        plusSpillCostInfo,

        slurpSpillCostInfo,
        chooseSpill,

        lifeMapFromSpillCostInfo
) where
import RegAlloc.Liveness
import Instruction
import RegClass
import Reg

import GraphBase

import BlockId
import Cmm
import UniqFM
import UniqSet
import Digraph          (flattenSCCs)
import Outputable
import Platform
import State

import Data.List        (nub, minimumBy)
import Data.Maybe


-- | Records the expected cost to spill some regster.
type SpillCostRecord
 =      ( VirtualReg    -- register name
        , Int           -- number of writes to this reg
        , Int           -- number of reads from this reg
        , Int)          -- number of instrs this reg was live on entry to


-- | Map of `SpillCostRecord`
type SpillCostInfo
        = UniqFM SpillCostRecord


-- | An empty map of spill costs.
zeroSpillCostInfo :: SpillCostInfo
zeroSpillCostInfo       = emptyUFM


-- | Add two spill cost infos.
plusSpillCostInfo :: SpillCostInfo -> SpillCostInfo -> SpillCostInfo
plusSpillCostInfo sc1 sc2
        = plusUFM_C plusSpillCostRecord sc1 sc2


-- | Add two spill cost records.
plusSpillCostRecord :: SpillCostRecord -> SpillCostRecord -> SpillCostRecord
plusSpillCostRecord (r1, a1, b1, c1) (r2, a2, b2, c2)
        | r1 == r2      = (r1, a1 + a2, b1 + b2, c1 + c2)
        | otherwise     = error "RegSpillCost.plusRegInt: regs don't match"


-- | Slurp out information used for determining spill costs.
--
--   For each vreg, the number of times it was written to, read from,
--   and the number of instructions it was live on entry to (lifetime)
--
slurpSpillCostInfo :: (Outputable instr, Instruction instr)
                   => Platform
                   -> LiveCmmDecl statics instr
                   -> SpillCostInfo

slurpSpillCostInfo platform cmm
        = execState (countCmm cmm) zeroSpillCostInfo
 where
        countCmm CmmData{}              = return ()
        countCmm (CmmProc info _ _ sccs)
                = mapM_ (countBlock info)
                $ flattenSCCs sccs

        -- Lookup the regs that are live on entry to this block in
        --      the info table from the CmmProc.
        countBlock info (BasicBlock blockId instrs)
                | LiveInfo _ _ (Just blockLive) _ <- info
                , Just rsLiveEntry  <- mapLookup blockId blockLive
                , rsLiveEntry_virt  <- takeVirtuals rsLiveEntry
                = countLIs rsLiveEntry_virt instrs

                | otherwise
                = error "RegAlloc.SpillCost.slurpSpillCostInfo: bad block"

        countLIs _      []
                = return ()

        -- Skip over comment and delta pseudo instrs.
        countLIs rsLive (LiveInstr instr Nothing : lis)
                | isMetaInstr instr
                = countLIs rsLive lis

                | otherwise
                = pprPanic "RegSpillCost.slurpSpillCostInfo"
                $ text "no liveness information on instruction " <> ppr instr

        countLIs rsLiveEntry (LiveInstr instr (Just live) : lis)
         = do
                -- Increment the lifetime counts for regs live on entry to this instr.
                mapM_ incLifetime $ nonDetEltsUFM rsLiveEntry
                    -- This is non-deterministic but we do not
                    -- currently support deterministic code-generation.
                    -- See Note [Unique Determinism and code generation]

                -- Increment counts for what regs were read/written from.
                let (RU read written)   = regUsageOfInstr platform instr
                mapM_ incUses   $ catMaybes $ map takeVirtualReg $ nub read
                mapM_ incDefs   $ catMaybes $ map takeVirtualReg $ nub written

                -- Compute liveness for entry to next instruction.
                let liveDieRead_virt    = takeVirtuals (liveDieRead  live)
                let liveDieWrite_virt   = takeVirtuals (liveDieWrite live)
                let liveBorn_virt       = takeVirtuals (liveBorn     live)

                let rsLiveAcross
                        = rsLiveEntry `minusUniqSet` liveDieRead_virt

                let rsLiveNext
                        = (rsLiveAcross `unionUniqSets` liveBorn_virt)
                                        `minusUniqSet`  liveDieWrite_virt

                countLIs rsLiveNext lis

        incDefs     reg = modify $ \s -> addToUFM_C plusSpillCostRecord s reg (reg, 1, 0, 0)
        incUses     reg = modify $ \s -> addToUFM_C plusSpillCostRecord s reg (reg, 0, 1, 0)
        incLifetime reg = modify $ \s -> addToUFM_C plusSpillCostRecord s reg (reg, 0, 0, 1)


-- | Take all the virtual registers from this set.
takeVirtuals :: UniqSet Reg -> UniqSet VirtualReg
takeVirtuals set = mkUniqSet
  [ vr | RegVirtual vr <- nonDetEltsUFM set ]
  -- See Note [Unique Determinism and code generation]


-- | Choose a node to spill from this graph
chooseSpill
        :: SpillCostInfo
        -> Graph VirtualReg RegClass RealReg
        -> VirtualReg

chooseSpill info graph
 = let  cost    = spillCost_length info graph
        node    = minimumBy (\n1 n2 -> compare (cost $ nodeId n1) (cost $ nodeId n2))
                $ nonDetEltsUFM $ graphMap graph
                -- See Note [Unique Determinism and code generation]

   in   nodeId node


-------------------------------------------------------------------------------
-- | Chaitins spill cost function is:
--
--   cost =     sum         loadCost * freq (u)  +    sum        storeCost * freq (d)
--          u <- uses (v)                         d <- defs (v)
--
--   There are no loops in our code at the momemnt, so we can set the freq's to 1.
--
--  If we don't have live range splitting then Chaitins function performs badly
--  if we have lots of nested live ranges and very few registers.
--
--               v1 v2 v3
--      def v1   .
--      use v1   .
--      def v2   .  .
--      def v3   .  .  .
--      use v1   .  .  .
--      use v3   .  .  .
--      use v2   .  .
--      use v1   .
--
--           defs uses degree   cost
--      v1:  1     3     3      1.5
--      v2:  1     2     3      1.0
--      v3:  1     1     3      0.666
--
--   v3 has the lowest cost, but if we only have 2 hardregs and we insert
--   spill code for v3 then this isn't going to improve the colorability of
--   the graph.
--
--  When compiling SHA1, which as very long basic blocks and some vregs
--  with very long live ranges the allocator seems to try and spill from
--  the inside out and eventually run out of stack slots.
--
--  Without live range splitting, its's better to spill from the outside
--  in so set the cost of very long live ranges to zero
--
{-
spillCost_chaitin
        :: SpillCostInfo
        -> Graph Reg RegClass Reg
        -> Reg
        -> Float

spillCost_chaitin info graph reg
        -- Spilling a live range that only lives for 1 instruction
        -- isn't going to help us at all - and we definitely want to avoid
        -- trying to re-spill previously inserted spill code.
        | lifetime <= 1         = 1/0

        -- It's unlikely that we'll find a reg for a live range this long
        -- better to spill it straight up and not risk trying to keep it around
        -- and have to go through the build/color cycle again.
        | lifetime > allocatableRegsInClass (regClass reg) * 10
        = 0

        -- Otherwise revert to chaitin's regular cost function.
        | otherwise     = fromIntegral (uses + defs)
                        / fromIntegral (nodeDegree graph reg)
        where (_, defs, uses, lifetime)
                = fromMaybe (reg, 0, 0, 0) $ lookupUFM info reg
-}

-- Just spill the longest live range.
spillCost_length
        :: SpillCostInfo
        -> Graph VirtualReg RegClass RealReg
        -> VirtualReg
        -> Float

spillCost_length info _ reg
        | lifetime <= 1         = 1/0
        | otherwise             = 1 / fromIntegral lifetime
        where (_, _, _, lifetime)
                = fromMaybe (reg, 0, 0, 0)
                $ lookupUFM info reg


-- | Extract a map of register lifetimes from a `SpillCostInfo`.
lifeMapFromSpillCostInfo :: SpillCostInfo -> UniqFM (VirtualReg, Int)
lifeMapFromSpillCostInfo info
        = listToUFM
        $ map (\(r, _, _, life) -> (r, (r, life)))
        $ nonDetEltsUFM info
        -- See Note [Unique Determinism and code generation]


-- | Determine the degree (number of neighbors) of this node which
--   have the same class.
nodeDegree
        :: (VirtualReg -> RegClass)
        -> Graph VirtualReg RegClass RealReg
        -> VirtualReg
        -> Int

nodeDegree classOfVirtualReg graph reg
        | Just node     <- lookupUFM (graphMap graph) reg

        , virtConflicts
           <- length
           $ filter (\r -> classOfVirtualReg r == classOfVirtualReg reg)
           $ nonDetEltsUFM
           -- See Note [Unique Determinism and code generation]
           $ nodeConflicts node

        = virtConflicts + sizeUniqSet (nodeExclusions node)

        | otherwise
        = 0


-- | Show a spill cost record, including the degree from the graph
--   and final calulated spill cost.
pprSpillCostRecord
        :: (VirtualReg -> RegClass)
        -> (Reg -> SDoc)
        -> Graph VirtualReg RegClass RealReg
        -> SpillCostRecord
        -> SDoc

pprSpillCostRecord regClass pprReg graph (reg, uses, defs, life)
        =  hsep
        [ pprReg (RegVirtual reg)
        , ppr uses
        , ppr defs
        , ppr life
        , ppr $ nodeDegree regClass graph reg
        , text $ show $ (fromIntegral (uses + defs)
                       / fromIntegral (nodeDegree regClass graph reg) :: Float) ]